When a fault or short-circuit occurs in an electric power system, a short and intense current flows. This is termed the “fault current” and it may be 20 to 50 times the usual operating current. These fault currents damage equipment and cabling. A superconducting fault current limiter uses an inherent property of superconductors to limit the fault current to a much lower level, typically 10 times the operating current.
FCLs are a well-known application of high temperature (copper-oxide ceramic) superconductors operating in liquid nitrogen. Almost without exception, existing FCLs use liquid boiling as a means of removing heat from the superconductor. A typical example is the device being produced by Nexans SuperConductors for the United States Department of Energy development project that began in 2003.
There is one design which uses gas cooling: “A Nitrogen Gas Cooled, Hybrid, High Temperature Superconducting Fault Current Limiter” by M. Steurer, H. Brechna and K. Frohlich of the Swiss Federal Institute of Technology ETH, CH-8092 Zurich, Switzerland, IEEE Trans. Appl. Supercond., Vol. 10, No. 1, pages 840-844, 2000. However this design essentially requires a “fast acting load switch” in parallel with the superconducting element. Thus the superconducting element in their design is not the element that detects and triggers the current limiting behaviour, only actuates it on being triggered, and their design has inadequate cooling capability for continuous operation of the superconducting element.
There are also non-superconducting FCLs based on entirely different silicon thyristor technologies.
FCLs have been designed for both direct current (DC) and alternating current (AC) operation. The most common use is anticipated to be in AC power systems operating at 60 Hz or 50 Hz. Aircraft systems at 400 Hz are also possible. In an AC system, the fault that creates the highest fault current is one that occurs when the AC half-cycle is near its maximum. The rise time for a half-cycle for a 50 Hz AC power system is 5 milliseconds. This is typically taken as the fault initiation time for FCL design as the inductance of the system being protected prevents much faster rise times.
The recovery time for an FCL is typically longer than that of the fault initiation time. Recovery times of between 5 milliseconds and several hours have been discussed in the published literature. On recovery, the FCL carries the usual operating current.
The fault repeat time is the time after recovery before an FCL can handle another fault without damage or without creating a permanent open circuit limited only by the external shunt and the internal normal resistance.
Many designs of FCL in the published literature have a limit to the number of faults they can handle within a period of hours or days without damage or without creating a permanent open circuit. This is equivalent to the recovery time increasing towards infinity or the time to repair.